When advertising OSPF routes into BGP, the appropriate routing policy should be applied as an export policy in BGP.
Step-by-Step Breakdown:
OSPF to BGP Route Advertisement:
Routes learned via OSPF (a dynamic IGP) need to be exported into BGP to be advertised to external BGP peers. In Junos OS, this is done using export policies.
Export Policies in BGP:
An export policy controls which routes are advertised out of a BGP session. In this scenario, the routing policy must be applied to BGP as an export policy to export the OSPF-learned routes to external BGP peers.
Policy Configuration:
Example configuration:
set policy-options policy-statement EXPORT_OSPF term 1 from protocol ospf set policy-options policy-statement EXPORT_OSPF term 1 then accept set protocols bgp group <group-name> export EXPORT_OSPF This policy ensures that only OSPF routes are exported into BGP.
Juniper Reference:
Routing Policy: Export policies are used in BGP to control route advertisements to peers, including those learned via OSPF.

Aggregate routes and generated routes are used to create summarized routes in Junos, but they behave differently in terms of forwarding.
Step-by-Step Breakdown:
Aggregate Routes:
An aggregate route summarizes a set of more specific routes, but it does not have a direct forwarding next hop. Instead, it points to the more specific routes for actual packet forwarding.
Generated Routes:
A generated route also summarizes specific routes, but it has a forwarding next hop that is determined based on the availability of contributing routes. The generated route can be used to directly forward traffic.
Juniper
Reference: Aggregate and Generated Routes: In Junos, aggregate routes rely on more specific routes for forwarding, while generated routes can forward traffic directly based on their next-hop information.